Television system



March 1, 1938. R. s. HOLMES TELEVISION SYSTEM Filed Deo. l, 1935 Patented Mar. 1, 1938 UNITED STATES PATENT OFFICE TELEVISION SYSTEM of Delaware Application December l, 1933, Serial No. 700,478

7 Claims.

My invention relates to improvements in television systems.

In communication by television, wherein picture signals and synchronizing signals are intercepted and supplied to some form of scanning apparatus which develops an image of the view at a particular transmitting station, there occur variations over a substantial range in the amplitude or level of the received signals. These variations occur, for example, upon tuning the receiving apparatus for reception from different transmitting stations of different power and/or different distances from the receiving apparatus. Such variations, also, are attributed to atmospheric disturbances and the phenomenon commonly referred to as fading.

The eiect of the variations referred to is manifested at the receiver by wide variations in the overall brilliancy of the received picture and, possibly, by overloading of the ampliilers.

With the foregoing in mind, it is one of the objects of my invention to provide an improved television system wherein provision is made for automatic control or adjustment of the same in such manner that the amplitude or level of the picture and synchronizing signals supplied to the scanning apparatus is maintained substantially constant.

Other objects and advantagesv will hereinafter appear.

In accordance with my invention, the intercepted picture and synchronizing signals, transmitted together on a carrier wave as modulations thereof, are passed through an electrical channel which supplies the scanning apparatus. The synchronizing signals are taken from this channel. at a point therein where they are of suillcient amplitude and of the right polarity, and are rectied to develop an electrical potential the amplitude of which varies proportionally with occurring variations in the effective amplitude of the synchronizing signals. The electrical potential so developed is applied to one or more control circuits in the electrical receiving channel, and is effective to decrease the sensitivity of the latter when the signal level tends to rise and to increase such sensitivity when the signal level tends to drop.

My invention resides in the improved system and method of operation of the character here- 'inafter described and claimed.

For the purpose ol' illustrating my invention, an embodiment thereof is shown in the drawing, wherein Figure 1 is a diagrammatic view of a television receiving system including a preferred embodiment of my invention;

Fig. 2 is a graphical illustration of the principle of operation in Fig. 1;

Fig. 3 is a diagrammatic View of an amplifier stage; and

Fig. 4 is a diagrammatic view, showing a modiiication of the system in Fig. 1.

With reference to Fig. 1, the numeral III designates scanning apparatus for developing an image of the transmitted subject. This apparatus is shown as being in the form f a cathode ray tube I2 provided with a gun I4 for developing a ray I6 o! electrons and directing the same at a iluorescent screen I8 on which the transmitted picture is reproduced. Assuming that there are to be 24 frames a second, each frame to have 180 lines horizontally, a plurality of electromagnetic coils 20 are provided, that operate to deilect the ray 43,20 times a second horizontally, and a plurality of electromagnetic coils 22 that operate to deflect the ray simultaneously 24 times a second vertically. For this purpose, a suitable generator 24 operates to develop a saw-tooth current wave at 4320 cycles and to cause this wave to pass through the coils 20, while a suitable generator 26 operates to develop a saw-tooth current wave at 24 cycles and to cause this wave to pass through the coils 22.

Operating action of the scanning apparatus is maintained in synchronism with that at f the transmitter by synchronizing or control signals eective to drive the generators 24 and 26, as will hereinafter more fully appear.

With regard to the various details of construction in Fig. 1, these are assumed to be comparable to those disclosed in the copending application by Arthur W. Vance, filed June 17, 1931 and bearing Serial No. 544,959.

For the purpose of reproducing the transmitted picture on the screen I8, the picture signals are applied to a control grid 28 forming part of the gun I4, whereby the intensity of the ray I6 is made tovary directly proportionally with occurring variations in the picture signals. A source 29 of anode potential is provided, and adjustment of potentiometer 30 determines the positive potential applied to the rst anode 32.

For the purpose of illustration, it shall be assumed that at the transmitter the object or picture is scanned to cause variation of current in the output circuit of some form of photosensitive apparatus, whereby picture signals in the form of a current wave are developed, as indicated at 34 in Fig. 2. Scanning action at the transmitter is the same as that at the receiver- That is, the object is scanned 24 times a second, and each scanning is made along 180 horizontal lines.

During about ninety per cent of each line period, the picture signals are developed, the amplitude at any instant of the picture current from the axis 36 being directly proportional to theI light intensity at that instant at the corresponding elemental area of the object being scanned. During the remaining period t of each line period, a synchronizing signal or impulse, indicated at 38 in Fig. 2, is developed by suitable ap- I paratus operating independently of the apparatus for developing the picture signals but synchronous with it. 'I'he synchronizing signals, therefore, occur at the rate of 4320 a second, and during the periods t, when there are no plcture signals. 'There are also developed, with the synchronizing signals, other control signals, indicated at 40 in Fig. 2, where are termed framing signals and occur at the rate of 24 a second.

The framing signals have the same amplitude as the synchronizing signals, but are of substantially greater duration than the latter. At the transmitter, also, the connections and operating action are such that the polarity of the picture signals, representative of conditions of light at the transmitted subject, is opposite to the polarity of the synchronizing and framing signals, as indicated in Fig. 2.

The picture signals and the synchronizing and framing signals, transmitted together on a carrier Wave as modulations thereof, are intercepted by an electrical channel A, having at'its input end aradio-frequency, alternating current amplifier 42.

'I'he incoming lcarrier wave is demodulated to a lower, intermediate frequency by a detector 44 and an associated local oscillator 46. The beat wave, at the intermediate frequency, is amplified by an intermediate frequency amplifier 48. 'I'he beat wave is then rectified by a second detector 50 at the output end of the channel A, to obtain the signal wave which is amplified by a low frequency amplifier 52 supplied from the second detector. y

In the output connection 54 from the amplifier 52, the signal wave and the polarities are as represented in Fig. 2, with the synchronizing signals in the black direction and of positive polarity.

The synchronizing and framing signals operate to drive the generators 24 and 26, respectively. The generator 24begins a new cycle of its operation with the occurrence of each synchronizing signal. The generator 26, in like manner, begins a new cycle of its operation upon the occurrence of each framing signal, and this generator is so constructed and adjusted that it will not respond to the synchronizing signals which are of substantially shorter duration than the framing signals.

The construction and adjustment of the generators 24 and 26 is such that they are not affected by the peak picture signals, but only by the control signals of greater amplitude.

'I'he signal wave, as represented in Fig 2, is applied by the connection 56 to the input of the picture signal circuit 58. From the output of this circuit, the picture signals representative of conditions of light on the transmitted subject are of positive polarity, and the synchronizing and framing signals are of negative polarity. 'I'hese signals are applied as indicated to the grid 28 of the tube I2. 'I'he amplitude of these signals is sufficiently high to completely cut 01T the ray I6 during return deflection thereof in the horizontal and vertical directions.

For the purpose of providing for the automatic control referred to above, the signal wave, as represented in Fig. 2, is supplied by the connection 56 to the grid circuit of a plate circuit detector 60. 'I'he peak amplitude :t of the synchronizing signals is a measure of, or is proportional to the strength of the incoming signals. This condition is utilized to vary the plate current of the tube 60 automatically and substantially proportional to variations in the signal strength. The result is that across a resistor 59 in the output or plate circuit of the tube 60, an electrical potential is developed whose amplitude varies substantially proportionally with occurring variations in the amplitude :c of the synchronizing signals. 'Ihat is, the anode end of the resistor becomes more negative with an in-l crease in .'c, and when :c decreases it becomes correspondingly less negative. The electrical potential so developed is applied to the grid circuit of one or more of the stages of the intermediate frequency amplier 48, as indicated by the connections 64 and as shown in Fig. 3. As the electrical potential becomes more negative, the sensitivity of the electrical channel is decreased correspondingly, and as the potential becomes less negative, the sensitivity of the channel is increased correspondingly. In this manner, the level of the signals applied to the scanning apparatus I is maintained substantially constant. For the same purpose, the negative control potential developed in the plate circuit of the tube 60 may be applied to the grid circuit of one or more stages of the radio frequency amplifier 42, as indicated by the broken lines 66.

For the purpose of normally biasing the grids of the controlled tubes to the best operating potential, a potential source 61 is provided, the positive terminal of which is connected to ground and the negative terminal of which is connected to the cathode end of the output resistor 56.

By adjusting the bias applied to the grid of the tube 60 from a source 68 the signal amplitude at which the control becomes effective may be determined.

In the modification shown in Fig. 4, the picture and synchronizing signals are fed directly to the picture signal circuit 58 from the low frequency amplifier 52, as in Fig. 1. For the purpose of the automatic control action, however, the signals, as represented in Fig. 2, are first taken from the low frequency amplifier 52 and passed through an auxiliary amplifier 1l of a conventional design to pass only the synchronizing and framing signals in Fig. 2, rejecting the picture signals of opposite polarity. The ampliiler supplies a suitable form of separating circuit 12 which operates to separate the synchronizing and framing signals, so that in the output connection 14 from this circuit there will appear only the synchronizing signals, and in the output connection 16 there will appear only the framing signals. The generator 24 is, therefore, supplied only with synchronizing signals by the connection 14, andthe generator 26 is supplied only with framing signals by the connection 16.

The control signals for the input circuit of the tube 60 can be supplied from the connection 18, or from the connection 80, or from either of the connections 14 and 16. In doing this, it will be understood that the conditions of polarity must be correct so that the desired control of sensitivity of the channel A is obtained.

In lieu of the tube 80, it will be understood that other known devices, such as a diode or a gridleak rectier, can be used for the same purpose, in which case consideration will be given for the correct conditions of polarity of the signals supplied to the rectier device, as will be well understood.

From the foregoing, it will be seen that I have provided an improved'television system wherein the receiving apparatus is controlled automatically to maintain the level of the picture and control signals substantially constant irrespective of occurringr atmospheric disturbances, tuning from one station to another, and the occurrence of any electrical effects which might tend to cause this level to vary, and that for this purpose the synchronizing signals are used.

While but one embodiment of my invention has been disclosed, it will be understood that various modications are possible without departing from the spirit of my invention or the scope of the claims.

I claim as my invention:

1. In a television receiving system, apparatus for developing an image of the transmitted subject, an electrical channel for intercepting picture signals and synchronizing signals transmitted together on a carrier wave as modulations thereof, a rectifier at the output end of said channel, an amplifier supplied from said rectifier and operating to supply said apparatus directly with picture signals and synchronizing signals, the picture signals representative of conditions of light at the transmitted subject being of opposite polarity both to the synchronizing signals and to the picture signals representative of conditions of shade at the transmitted subject, means supplied from said amplifier for developing an electrical potential whose amplitude variesin response to and in accordance with occurring variations in the effective amplitude of the synchronizing signals, and connecting means for applying such electrical potential to an operating circuit in said channel effective to control the sensitivity of said channel in accordance with occurring variations in amplitude of the applied potential. I

2. In the art of television, the method of operation Which comprises transmitting picture signals and synchronizing signals together on a carrier wave as modulations thereof and under conditions such that the picture signals representative of conditions of light at the transmitted subject are of opposite polarity both to the synchronizing signals and to the picture signals representative of conditions of shade at the transmitted subject, utilizing the received synchronizing signals to develop an electrical potential whose amplitude varies in response to and in accordance with occurring variations in the effective amplitude of the synchronizing signals, and applying the developed electrical potential to the receiving apparatus to vary the sensitivity thereof in the sense that an increase in the amplitude of the received synchronizing signals causes a decrease in the sensitivity of said apparatus and a decrease in the amplitude of the received synchronizing signals causes an increase in the sensitivity of said apparatus.

3. In a television receiving system, apparatus for developing an image of the transmitted subject, an electrical channel for intercepting picture signals and synchronizing signals transmitted together on a carrier wave as modulations thereof;

said channel comprising an amplifier at the input end thereof, a detector supplied from said amplifler, a second amplifier supplied from said detector, and a second detector supplied from said second amplifier; a third amplifier supplied from said second detector and operating to supply picture and synchronizing signals directly to said apparatus under conditions such that the picture signals representative of conditions of light at the transmitted subject are of opposite polarity both to the synchronizing signals and to the picture signals representative of conditions of shade at the transmitted subject, means supplied from said third amplifier and operating to develop an electrical potential of negative polarity which becomes more negative upon an increase in the effective amplitude of the synchronizing signals and which becomes less negative upon a decrease in such amplitude, and connecting means for applying such electrical potential to at least one control circuit in said channel effective to vary the sensitivity of the latten 4. In a television receiving system, a cathode ray tube for developing an image of the transmitted subject and provided with a screen and with means for developing a ray of electrons and directing the ray at said screen, means for causing the ray to scan a given area of said screen and comprising means for deflecting the ray in a given direction at a given frequency, an electrical channel for intercepting picture signals and control signals transmitted together on a carrier Wave as modulations thereof, means supplied with picture signals from said channel and operating to vary the intensity of said ray in accordance with occurring variations in such signals, said ray-deflecting means being supplied with the control signals from said channel and being maintained thereby in operation at said frequency, means supplied from said channel with an electrical wave comprising the picture and control signals for developing an electrical effect which becomes more negative in response to an increase in the effective amplitude of the control signals and which becomes less negative in response to a decrease in such amplitude, and connecting means for applying such electrical effect to at least one control circuit in said channel effective to vary the sensitivity of the latter.

5. In a television receiving system, a cathode ray tube for developing an image of the transmitted subject and provided with a screen and with means for developing a ray of electrons and directing the ray at said screen, means for causing the ray to scan a given area of said screen and comprising means for deecting the ray in a given direction at a given frequency, an electrode for Varying the intensity of the ray, an electrical channel for intercepting picture signals and control signals transmitted together on a carrier wave as modulations thereof; said channel comprising an amplifier at the input end thereof, a detector supplied from said amplifier. a second amplifier supplied from said detector, and a second detector supplied from said second amplifier; a third amplifier supplied from said second detector and operating to supply the picture and control signals to said electrode under conditions such that the picture signals representative of conditions of light at the transmitted subject are of opposite polarity both to the control signals and to the picture signals representative of conditions of shade at the transmitted subject, means supplied from said third amplier and operating to develop an electrical eiect which becomes more negative upon an increase in the effective amplitude of the control signals and which becomes less negative upon a decrease in such amplitude, and connecting means for applying such electrical eiect to at least one control circuit in said channel eiective to vary the sensitivity of the latter.

6. In` a television system ofthe type wherein a view is scanned at the transmitter and a screen is scanned at the receiver and in which picture signals and synchronizing signals are transmitted as a composite signal and in which the picture signal voltage representative of conditions of light is of opposite polarity to the synchronizing signal voltage with respect to the alternating current axis of said composite signal, a receiver comprising an electrical discharge device having a control electrode, means for deriving a unidirectional control voltage from` said composite signal which voltage varies substantially in accordance with the amplitude of said synchronizing signals as measured from said axis, means for applying said control voltage to said control electrode whereby the operation of said electric discharge device iscontrolled in accordance with rthe amplitude of said synchronizing signals, and

means for utilizing said synchronizing signals for maintaining the scanning at the receiver' in synchronism with the scanning at the transmitter.

7. In a. television system of the type in which picture signals and synchronizing signals are transmitted as a composite signal and in which the picture signal voltage representative of conditions of light is of opposite polarity to the synchronizing signal voltage with respect to the `alternating current axis of said composite signal, a receiver comprising a signal channel including an amplier for amplifying a carrier modulated by said composite signal and a detector located at a later point in said channel for reproducing said composite signal, said ampliiler including a gain control means, means for deriving a uni-directional control voltage from 

